Matrix displays in which an image is formed by selective activation of a matrix of display electrodes, each constituting one picture element of the display, are generally addressed by means of orthogonal sets of row and column lines. To display a full range of images, it is necessary to employ time multiplexing techniques and to write such displays one row or column at a time, with the image data to be written on the selected row or column being presented concurrently on the orthogonal set of lines.
Passive matrix addressing, in which the contrast of the picture element is directly determined by the electrical waveforms present on the coordinate row and column lines, is only feasible with certain types of liquid crystal material. Such materials must have a relatively sharp optical transmission characteristic so that an observable contrast difference exists between elements receiving signals on both coordinate lines and elements receiving signals on only one line. Additionally the liquid crystal material must have sufficient persistence or memory to remain in the contrasting state until the whole matrix has been written and a refresh operation can commence.
Many liquid crystals do not have such memory properties and these can only be addressed by means of an active scheme in which each display electrode has an associated transistor switch and storage capacitor. The transistor switch is opened by a signal on one address line to permit charging or discharging of the capacitor to the voltage on the orthogonal address line. If the liquid crystal is of the d.c. driven type, the voltage on the capacitor may be sufficient to control the contrast state of the liquid crystal picture element directly. Such a display is described in an article by K. Kasahara et al., entitled "A liquid-crystal TV display panel" using a MOS array with gate-bus drivers (Conference Record of the 1980 Biennial Display Research Conference, pp 96-101, published by the IEEE). To improve the fabrication process yield, dual gate bus driver arrays are shown but are not used simultaneously. One only is enabled by a switch circuit to limit power consumption.
Another liquid crystal display, using a.c. drive techniques, is described in an article entitled "A pocket sized liquid crystal TV display" by E. Kaneko et al., (S.I.D. '81 Digest pp 84-85). In this case the vertical line drive circuits are split into two with drivers at the top and bottom of the array driving interleaved lines.
If the liquid crystal is not of the d.c. type, as per Kasahara et al, or the a.c. type of relatively low pel density as described in the Kaneko paper the capacitor may be used to control a second transistor which connects a driving waveform, e.g. an a.c. waveform, to the associated liquid crystal display electrode. Such a scheme is discussed in a paper by D. J. Barclay et al., entitled "The Design of Silicon based passive displays" from Electronics to Microelectronics, W. A. Kaiser and W. E. Proebster (Eds.) pp 737-740, published by North Holland). The Barclay et al. paper discusses comparatively the problems of matrix addressing both liquid crystal and electrochromic displays whose display electrodes and addressing circuits are integrated on a silicon chip.
Because an electrochromic display has no well defined contrast threshold, it is necessary to address it by means of the active technique, i.e., via switching transistors connected to each display electrode. However, because of the persistence of the electrochromic effect after removal of the applied write voltage, no storage capacitor is needed and only a single switching transistor is required. Thus the circuit elements associated with each display electrode are much simpler than in the liquid crystal case.
The writing of an electrochromic display is performed similarly to a liquid crystal matrix display, i.e. by one line at a time multiplexing. However, as a well known alternative to voltage driving in electrochromic displays, such as those of the viologen type, where the resulting uncontrolled cathodic potentials at the display electrodes do not run the risk of producing adverse side reactions, constant current writing is preferred. Constant current writing has the advantage of speed and can easily be made synchronous with the line scanning operation since application of current for a fixed interval corresponds to deposition of a fixed charge on the display electrode.
Erasure of electrochromic displays is, however, not analogous to the liquid crystal case. With passively addressed liquid crystal displays the image simply disappears when the applied voltage is removed. Similarly, with actively addressed liquid crystal displays, whenever the voltage across the liquid crystal picture element (pel) is removed, i.e. the charge on the capacitor is removed either actively and rapidly or more slowly by leakage current discharge, the liquid crystal pel image disappears quickly. Electrochromic displays have stored charges of the order of 2 mCcm.sup.-2 and since the charge is often in the form of a low conductivity physical deposit, leakage is relatively slow.
For this reason, the charge stored on the electrochromic display electrodes must be positively removed by reverse current flow. To achieve this, the technique of potentiostatic erasure is preferred. This technique, which is well known, necessitates a reference electrode in contact with the electrolyte of the electrochromic cell. An erase voltage source monitors the potential of the reference electrode with respect to the solution and develops an erase voltage which is fixed with respect to the reference electrode potential. The erase voltage corresponds to the potential of an unwritten display electrode in solution. It is applied to the display electrodes to be erased and erase current flows to remove the charge on the display electrodes until they, too, are at the erase voltage. The technique has the advantage that display electrodes are not overdriven into undesirable side reactions (e.g., anodisation). This could not be guaranteed with a constant current or even a constant voltage technique. Addressing for erasure is simplified by use of the potentiostatic technique because no harm is done if unwritten electrodes are connected to the erase voltage source. This enables block erasure of part or the whole of the display by selection of multiple lines from each of the orthogonal sets. Line-by-line multiplexing is not necessary.
One such electrochromic matrix display employing constant current writing and potentiostatic erasure is shown in U.S. Pat. No. 4,426,643. This shows a viologen based display in which the display electrodes, a matrix of transistor switches row and column addressing lines and drive circuitry are all integrated on a silicon chip. Row selection information is fed into a row select shift register the outputs of which cause an array of row drivers to drive the selected rows to gate "on" the transistor switches of those rows. Similarly, column selection information representing one line of image data is shifted into a column shift register. An associated column driver array is responsive to the contents of the column shift register to apply either write or erase current to the selected column lines.
Another similar electrochromic matrix display is shown in copending U.S. application Ser. No. 626,505. This application shows a technique of monitoring the total erase current flow in the potentiostatic erase operation and providing an indication that erasure is complete after the erase current has fallen sharply to zero. In this way, excessive worst-case times, sufficient to allow erasure of a complete screen with every electrode written, need not be allowed for the completion of the essentially asynchronous potentiostatic erase process. Instead an "erase complete" signal produced at the actual completion of each particular erase operation allows the next display operation to proceed.